Radome constructions, or simply radomes, are highly electromagnetically transparent structures used for covering and protecting antennas.
Antennas and in particular large antennas such as radar installations, wireless telecom infrastructure and radio telescopes often need a radome or a covering structure of some kind to protect them from weather, e.g. sunlight, wind and moisture. The presence of the radome is particularly mandatory for antennas placed in regions where high winds or storms often occur, in order to protect the antennas from hale and impacts from projectiles such as debris carried by the wind.
Radome designs usually address structural requirements, including aerodynamic shape, rigidity, and resistance to weather, shock, impact, vibrations and biodegradations, as well as electromagnetic transparency, e.g. a minimal reflection and/or absorption of passing electromagnetic energy, with the aim of minimizing an electromagnetic energy loss.
Structural requirements for radomes are usually met by using composite materials having suitable mechanical properties when constructing a radome and in particular a radome wall. However, the known composite materials tend to have inadequate electromagnetic properties, e.g. a rather high dielectric constant and a rather high dielectric loss, especially at high and ultrahigh frequencies. It was also noticed that the known composite materials may have inadequate electromagnetic transparency and in particular a high electromagnetic absorption and/or being highly reflective.
An example of a radome manufactured from a composite material having suitable mechanical properties is discloses in an article by H. P. J. de Vries, i.e. “Design, fabrication and testing of a Dyneema®/polyethylene radome for airborne remote sensing”, Nationaal Lucht-en Ruimtevaartlaboratorium, NLR, Amsterdam, The Netherlands, 1 Jan. 1998, Amsterdam, The Netherlands. Therein a radome comprising a composite material containing low density polyethylene (LDPE) films and a stack of fabrics made from Dyneema® fibers is disclosed, the radome having a loss tangent of 0.0002 (2e-4). Although unclear at which electromagnetic frequency the loss tangent was measured, the present inventors analyzed such a radome and determined that for high and ultrahigh frequencies, the electromagnetic properties and in particular the electromagnetic transparency thereof can still be improved.
A further disclosure of radomes based on composite materials containing polyethylene fibers is given in “Electrical properties of polyethylene fiber composites” by Chia-Lun J. Hu et al, 31st International SAMPE Symposium, Apr. 7-10, 1986. Therein it is reported a radome comprising a composite material containing Spectra 900 fibers and an epoxy matrix and having a loss tangent measured at 10 MHz of about 1e-4. It was however observed that at frequencies of above 1 GHz (1000 MHz) the electromagnetic transparency of such a radome is rather poor. This is shown in “Spectra® reinforced composite systems for high impact/microwave transparent radar domes” by David S. Cordova et al., 2nd International SAMPE Electronics Conference, Jun. 14-16, 1988, which investigates the behavior of such composite materials at frequencies in the X-band. Therein it is shown that a radome based on Spectra® fibers and epoxy matrices has a loss tangent as measured in the X-band of about 0.0044 (4.4e-3).
A further investigation on the behavior of radomes made of Spectra® fibers embedded in an epoxy radome at frequencies in the W-band, i.e. higher than X-band, is given by a NASA Technical memorandum (110344) entitled “Complex Permittivities of Candidate Radome Materials at W-band” by Robin L. Cravey, May 1997. Therein it is shown that such a radome has a loss tangent as measured in the W-band of about 0.02 (2e-2).
Polyolefins other than polyethylenes can also be used in composite materials suitable to manufacture radomes thereof. An example of a radome made from self reinforced polypropylene is given in EP 1 1852 938, the radome having a loss tangent as measured at 10 GHz of about 0.0015 (1.5 e-3). However, such radomes usually have poorer electromagnetic properties than those using polyethylenes.
To improve however the electromagnetic properties of a radome and in particular its electromagnetic transparency, several solutions were proposed by various inventors. One approach to minimize in a material the reflection of passing electromagnetic energy and thus to minimize the electromagnetic energy loss, also referred to as electromagnetic signal loss or simply as signal loss, is to choose a certain material thickness, or in case of a radome, a certain radome wall thickness. Said thickness is typically adjusted with due regard to the electromagnetic properties of the material as well as to the characteristics of an electromagnetic signal interacting therewith, e.g. number of energy or signal beams, switching speeds, angles of incidence, said signal's wavelength or frequency. One particularly preferred solution is to adjust said thickness with due regard to the dielectric constant of the material and to the wavelength of the signal, so that waves reflected from a front surface of said material and from a back surface of the material cancel each other by destructive interference.
Another approach, to minimize in a material the reflection of passing electromagnetic energy, is to interpose on the surface of the material a quarter-wavelength thick layer having a refractive index intermediate that of air and that of the material. Yet another approach, to minimize in a material the reflection of passing electromagnetic energy, is by using a composite material containing a foamed plastic material.
Various composite materials and radomes with radome walls containing thereof and designed using at least one of the hereinabove mentioned approaches are to be found in U.S. Pat. No. 4,980,696; GB 846 868; U.S. Pat. Nos. 4,590,027; 4,783,666; and 5,059,972. However, the composite materials used therein for constructing the radome walls, even when carefully designed to minimize the reflection of passing electromagnetic energy, function optimally only for an electromagnetic energy at a matched wavelength. Moreover, the allowable energy loss throughout the radome wall limits their maximum thickness to relatively thin radome walls, which in turn deleteriously influences their structural properties. In particular it was observed that thin radome walls may have insufficient structural integrity in particular for large size radomes, especially those utilized in conjunction with antennas operating at high frequencies in the range of microwave frequencies, i.e. above 1 GHz.
To minimize the absorption of electromagnetic energy in a material and in case of radomes in the walls thereof, low dielectric constant materials were used for the construction of the radome walls. However, due to the fact that low dielectric constant materials often do not meet the structural requirements, higher dielectric constant materials with better mechanical properties are used in conjunction therewith. Multi-layer materials and radome walls containing thereof were developed to compensate for the above mentioned drawbacks, examples being found in U.S. Pat. Nos. 4,613,350 ; 4,725,475 ; 4,677,443 ; 4,358,772 ; 3,780,374 ; 5,408,244; and 7,151,504.
However, designing materials having an optimum combination of electromagnetic and mechanical properties is difficult and especially inefficient for frequencies above 10 GHz since such materials may show a rather large electromagnetic signal loss.
Numerous design attempts to obtain materials having superior electromagnetic transparency and radomes containing thereof are found in U.S. Pat. No. 7,151, 504; U.S. 2004/021985; U.S. 2006/0255948; U.S. 2007/0039683; U.S. 2007/0292674; U.S. 2008/0187734; U.S. 2008/0188153; U.S. 2008/0252552; U.S. 2009/0148681; U.S. 2009/0167628; and U.S. 2009/0207095. Most of these materials usually meet the structural requirements for various radome applications and have rather good electromagnetic properties for a matched wavelength of the electromagnetic signal interacting therewith. Some materials may also be suitably used for constructing broadband radomes, i.e. radomes utilized in conjunction with broadband antennas. For example several broadband materials for broadband radomes are reported in US 2008/0187734, which achieved at a matched frequency of 100 MHz a minimum dielectric constant of about 3.05 and a minimum dielectric loss of about 0.0015 (1.5×10−3) radians. However, the known broadband materials are effective typically at frequencies lower than microwave frequencies and usually said materials present a large dielectric loss at microwave frequencies.
Although the known materials have been found somewhat suitable for constructing radomes for antennas functioning at frequencies of up to 100 GHz, the electromagnetic signal loss in the radome walls have been found to be rather large. In particular, many applications are presently being developed requiring a greater sensitivity than hitherto. It should be recognized that at high frequencies in the range of 1 GHz and above, and in particular at ultra-high frequencies in the range of 50 GHz and above, designing materials having efficient electromagnetic properties is extremely challenging. For instance, very few materials have electromagnetic properties, e.g. a dielectric constant and/or a dielectric loss that would enable the manufacturing of an effective radome for antennas operating at frequencies higher than 50 GHz and even higher than 70 GHz. When using known materials in a radome for an ultra-high frequency antenna, it was observed that the antenna had a short operating range and its power had to be drastically increased to compensate for any signal loss. Increasing the antenna's power may in turn reduce the antenna's operating lifetime and also increases the operating cost due to high electricity consumption.
Faced with the above drawbacks, an object of the invention may thus be to provide a material which would enable the manufacturing of an efficient broadband radome, i.e. a radome which shows a good electromagnetic transparency over a large bandwidth and in particular in the microwave bandwidth, e.g. for frequencies up to 140 GHz and more in particular for frequencies between 1 GHz and 130 GHz.
Another object of the invention may be to provide a material which would enable the manufacturing of an efficient broadband radome, said material having reduced dielectric loss over a large bandwidth and in particular for frequencies between 1 GHz and 130 GHz. The dielectric loss is also referred to herein as loss tangent and is expressed in radians.
Another object of the invention may be to provide a material which would enable the manufacturing of an efficient broadband radome, said material having reduced dielectric loss for certain frequencies within the frequency range of between 1 GHz and 130 GHz.
Another object of the invention may be to provide a material which would enable the manufacturing of an efficient broadband radome, said material having a reduced variation of the dielectric loss over a large bandwidth and in particular for frequencies between 1 GHz and 130 GHz.
Another object of the invention may be to provide a material, which would enable the manufacturing of an efficient broadband radome, said material having excellent mechanical properties and/or offering suitable impact protection
Another object of the invention may be to provide a method for making a material, said material being suitable for the manufacturing of an efficient broadband radome.